Validate input

re="^[0-9]+$"

read -r -p "Enter a digit: " digit

if [[ $digit =~ $re ]]
then
    echo "$digit is a digit."
else
    echo "$digit is NOT a digit."
fi

It is the operator =~ which says that we want to use a regular expression. It will work if we use new test: double [[.

[[ string =~ pattern ]]

[0-9]+ only matches numeric values ​​0-9, one or more times.
^ and $ makes sure that there are only numeric characters from beginning to end.

Sometimes you get a prompt asking the user to choose y or n for example:

Are you sure? [y/N]

In most cases, one of the options is uppercase and is the option that is selected if you just press Enter. The code behind the validation may look like this:

re="^[yY]|yes$"

read -r -p "Are you sure [y/N]: " answer

[[ $answer =~ $re ]] || exit 0

# The rest of the code...

Here the user can enter y, y or the word yes. If you do not do this, the program ends.

#Revision history